Marked genomic diversity of norovirus genogroup I strains in a waterborne outbreak

Marked norovirus (NoV) diversity was detected in patient samples from a large community outbreak of gastroenteritis with waterborne epidemiology affecting approximately 2,400 people. NoV was detected in 33 of 50 patient samples examined by group-specific real-time reverse transcription-PCR. NoV genotype I (GI) strains predominated in 31 patients, with mixed GI infections occurring in 5 of these patients. Sequence analysis of RNA-dependent polymerase-N/S capsid-coding regions (∼900 nucleotides in length) confirmed the dominance of the GI strains (n = 36). Strains of NoV GI.4 (n = 21) and GI.7 (n = 9) were identified, but six strains required full capsid amino acid analyses (530 to 550 amino acids) based on control sequencing of cloned amplicons before the virus genotype could be determined. Three strains were assigned to a new NoV GI genotype, proposed as GI.9, based on capsid amino acid analyses showing 26% dissimilarity from the established genotypes GI.1 to GI.8. Three other strains grouped in a sub-branch of GI.3 with 13 to 15% amino acid dissimilarity to GI.3 GenBank reference strains. Phylogenetic analysis (2.1 kb) of 10 representative strains confirmed these genotype clusters. Strains of NoV GII.4 (n = 1), NoV GII.6 (n = 2), sapovirus GII.2 (n = 1), rotavirus (n = 3), adenovirus (n = 1), and Campylobacter spp. (n = 2) were detected as single infections or as mixtures with NoV GI. Marked NoV GI diversity detected in patients was consistent with epidemiologic evidence of waterborne NoV infections, suggesting human fecal contamination of the water supply. Recognition of NoV diversity in a cluster of patients provided a useful warning marker of waterborne contamination in the Lilla Edet outbreak.